1. Field of the Invention
The present invention concerns a damping device which is particularly, however not exclusively, adapted for damping boring bars and similar toolholding bars which are used in machine tools. Typical applications for the invention are damping vibrations in boring bars used in metal cutting machining (turning, milling etc.), tools with extreme lengths in relation to tool diameter, or in machining on slender structural elements, slim shafts and so on. The self-generated vibrations or oscillations typically arise as a consequence of the dynamic stiffness of the toolholder, workpiece, or machine being too weak.
Similar types of damping devices are previously known, and a related device is disclosed in Norwegian Patent no. 128.725. This Norwegian patent relates to a damper body which necessarily must be placed in a bore in a boring bar, because the bore itself constitutes an essential part of the damping means, i.e. the damping effect is a result of an interaction between the damper body and the bore itself, via resilient elements located therebetween. Another related device is known from British Patent Specification No. 1,279,217, which patent shows a system where outside washers together with associated fixing elements etc., are fixedly connected to a damper body, and consequently will be forced to oscillate with the same frequency and amplitude as the damper body. Such an assembly will be prone to having problems at high speeds, due to the risk of opening of joints. Also, the assembly procedure seems too complicated with such a device. Furthermore, having one spring element in an end cutout of the damper body and one spring element external to the damper body, results in an asymmetrical configuration, which is unfortunate.
In modern machining in particular, as it is developing towards higher cutting speeds, resulting in higher temperatures at the cutting edge, there is a growing demand for tooling where the coolant can be lead all the way up to the cutting edge.
In damping of rotating components, it is most important that the damping device is able to tolerate a high velocity of rotation. For example, for rotating tools in modern machining, the development mentioned above means a very high velocity of rotation, and it is therefore of the greatest importance in these applications that the damping device by design can tolerate the highest velocity of rotation. This has been a problem when using the prior art damping devices.
Furthermore, usually the space for accomodating damping oil is, in the previously known damping systems, located between the damper body and a bore for the damper body inside the bulk of the boring bar. The volume of this space around the damper body can only be partially filled with oil, because if it is completely filled, the fluid will act on a much too big surface of the damper body, leading to a too heavy restriction on the movement of the damper body, and hence the damper device will not be able to provide the desired effect.
When forced to leave the above mentioned volume only partially filled with oil, one will experience that this causes functional problems of the prior art with partial or complete loss of damping effect if the distribution of the oil changes, for example when rotating the device, when altering the direction of the central axis of the damper out of a horizontal plane, or when there is variation in the directions in which the vibrations impinge onto the damping device.
Finally, with the damping oil in a partially filled volume outside the damper body, it will not be possible to change the damper body weight without also changing the volume of the oil space.